The invention relates to an electro-acoustic system for improving the acoustic of a predetermined room, said system comprising a microphone array having a plurality of microphones and a loudspeaker array having a plurality of loudspeakers, as well as a signal processing unit, interposed between said arrays, said signal processing unit having means for generating reflections.
Such an electro-acoustic system is known from the NAG-publication of the Nederlands Akoestisch Genootschap (Dutch Acoustic Society) No. 92, 1988, ACHTERGRONDEN, PRINCIPES EN TOEPASSINGEN VAN HET "ACOUSTICAL CONTROL SYSTEM" (ACS) (backgrounds, principles and applications of the "acoustical control system") by D. de Vries, D.Sc. and Prof. A. J. Berkhout, D.Sc., pp. 53-64 (also published in the journal of the Nederlands Elektronica en Radio Genootschap (Dutch Electronics and Radio Society) (1988)). This known electro-acoustic system will be called the ACS-system hereinafter. The ACS-system is installed in the auditorium of the Technische Universiteit (University of Technology) at Delft, The Netherlands, and in the Cultureel Centrum (Arts Centre) at Winterswijk, The Netherlands. Reference is also made to the journal Podium, Volume 6, Nos. 6 and 7, October and December 1988.
The ACS-system will be described in more detail hereinafter, referring in particular to FIGS. 4-6 and section 4 of page 59 of the above-mentioned NAG-publication. Instead of using acoustic feedback for producing reverberation, the ACS-system uses means for generating reflections, in particular a central processor. In principle, any desired reverberation time can be realized by the ACS-system, provided it is longer than that of the predetermined room. Said reverberation time is independent of the number of listeners in the predetermined room. In the ACS-system the aim is to keep the acoustic feedback as small as possible, in particular by firstly directing the microphones in such a manner that a great deal of direct sound and relatively little reflected sound is received from the sound source in the predetermined room; that is, in a room with a stage and an auditorium or an audience area, with a lot of microphones on or around the stage, whilst reflecting surfaces in the stage area are undesirable, whereby, in case the ACS-system is used in a theatre, it is advised to place the musicians between stage curtains of the stage and not to use any sound reflectors that may be present or a dismountable "orchestra shell", because this leads to interfering reflections. In the second place, acoustic feedback is reduced by using directional microphones. In the third place, acoustic feedback is minimized by directing the loudspeakers at the audience in the predetermined room. In the fourth place, acoustic feedback is reduced in the ACS-system by varying the time of the matrix-elements in the central processor.
Characteristic of the ACS-system is furthermore that a few dozens of microphones and loudspeakers are used on the stage and in the auditorium (the same number of microphones and loudspeakers in practice). The microphones above the stage are suspended low over the orchestra, i.e. about 4 meters. The usual number is 24-32 microphones with an equal number of loudspeakers. The acoustic parameters of the predetermined room itself are disregarded. The extent of the system is indenpendent of the desired degree of improvement with respect to the existing acoustic. It is necessary to use microphones directed at the stage and loudspeakers directed at the audience in the auditorium (also called "acoustic holography"), because the realization of a complete acoustic according to predetermined specifications is aimed at. The loudspeakers are optimally directed at the audience by building them into the ceiling of the auditorium, as well as into wall parts of the auditorium, which are directed at the audience in such a manner that no reflections are produced. As a result it is often difficult to realize lateral reflections, because loudspeakers placed on the side of the audience may lead to reflections from opposite walls.
Because the area of the stage lacks reflections, supporting reflections and reverberation will often be produced on the stage by a subsystem, the so-called "stage reflection module", which consists of a plurality of microphones in the auditorium and a plurality of loudspeakers on the stage, about 12 of each in practice, in order that the musicians can hear themselves and each other. The microphones in the auditorium which form part of said stage reflection module are located at a relatively short distance from the loudspeakers of the so-called "auditorium reverberation module". The microphones above the stage forming part of said auditorium reverberation module are located at a relatively short distance from the loudspeakers of the stage reflection module. In this way the two subsystems are interconnected, in the form of a kind of loop, by acoustic coupling. The oscillation limits of the two modules are coupled, therefore.
The signal from each microphone of the auditorium reverberation module or stage reflection module is supplied, via the central processor added thereto, to each loudspeaker amplifier of the module in question (the loudspeaker amplifiers or the power amplifiers may be considered to be incorporated in the loudspeaker device or the signal processing unit). As a result a module has only one oscillation limit, which is determined by the most critical microphone-microphone amplifier-loudspeaker amplifier-loudspeaker chain (the microphone amplifier, or the preamplifier, may be considered to be incorporated in the microphone array or the signal processing unit), whereby also the total feedback between the joint loudspeakers and microphones plays a role.
A hum of voices and ventilation noise, for example, can be amplified by the microphones suspended in the auditorium, 12 in number for example.
It remains to be seen whether the system is suitable for the lyric theatre, because in that case the microphones must be suspended higher, in view of the fact that scenery must be provided.
Essential for the ACS-system is that it is aimed at to have the settings of the system sound the same in every auditorium; that is, that the individual character of the auditorium is not used. Reflections presented to the listeners by the system only emanate from signals produced by one or more central processors, which implies that a completely artificial acoustic is generated, without making use of the properties of the auditorium itself, that is, simulation of a desired acoustic is realized by the ACS-system.
The object of the invention is to provide an electro-acoustic system for improving the acoustic of a room in which music can be performed by extending the reverberation time and by enhancing the spaciousness of the sound while maintaining the acoustic properties of said room, i.e. improvement insofar as is necessary.
In order to accomplish that objective the invention provides an electro-acoustic system of the kind mentioned above, characterized in that at least one of the microphones is directed in such a manner that it receives at least reflected sound from a sound source in the predetermined room and/or that at least one of the loudspeakers is directed at a reflecting surface in the predetermined room.
Said measures imply the following possibilities, which possiblities all have the common feature, however, that besides the electronic generation of reflections or the enhancement of the reflection density by the signal processing unit, acoustic reflections are generated or the reflection density is increased by suitably directing the microphones and/or the loudspeakers in accordance with one or more of the following arrangements:
In the first place the microphones are directed for receiving direct sound and the loudspeakers directed at reflecting surfaces.
In the second place the microphones are directed for receiving direct sound and reflected sound and the loudspeakers are directed at reflecting surfaces.
In the third place the microphones are directed for receiving direct sound and reflected sound and the loudspeakers are directed at listeners.
In the fourth place the microphones are directed for receiving reflected sound and the loudspeakers are directed at reflecting surfaces.
In the fifth place the microphones are directed for receiving reflected sound and the loudspeakers are directed at listeners.
It is noted that directing at least one of the microphones in such a manner that it receives at least reflected sound from a sound source in the predetermined room is known per se from the published text of the lecture delivered by D. Kleis, M.Sc. for the Nederlands Akoestisch Genootschap (Dutch Acoustic Society) at Eindhoven on Mar. 17 , 1976, entitled: "Een eenvoudig multikanaal ambiofoniesysteem" (A simple multichannel ambionophony system) by Prof. J. J. Geluk, D.Sc., Radio Nederland Wereldomroep Hilversum, The Netherlands, D. Kleis, M.Sc., Philips Elektro-Akoestiek Breda, The Netherlands, EHR60/3-004/76, 15 March 1976. (See also the literature mentioned in said text). This electro-acoustic system, known by the name of "Multiple-Channel Reverberation System", will be called the MCR-system hereinafter. Said MCR-system is inter alia installed in the Philips Ontspannings Centrum at Eindhoven, the Netherlands (90 channels). Reference is also made to the journal Podium & Techniek, Volume 3, No. 6, December 1981, pp. 14-15 and the publication Philips Technical Review, Volume 1983/84, No. 41, pp. 12-23.
The MCR-system is based on the generation of reverberation by acoustic feedback between microphones and loudspeakers, however. In particular this known system consists of a plurality of identical channels. Each channel is a microphone-amplifier-loudspeaker combination. The amplification of a channel can be adjusted such that the sound reproduced by the loudspeaker falls on the microphone with sufficient signal intensity to be reamplified; i.e. acoustic feedback. In this manner each channel delivers a number of reflections which are delayed in time with respect to one another and which become weaker and weaker. When the acoustic feedback is enhanced there may be coloring by selective frequency-dependent decay. When the amplification is set even higher, with a closed-loop gain larger than 1, the system becomes unstable and oscillation occurs. Because the allowable amplification per channel is small, also the extension of the reverberation time per channel is small. Generally it is assumed that, dependent on the coloring that is considered allowable, 50-100 channels are required in order to double the reverberation time of the auditorium itself. Each microphone is located in the reverberant field of the loudspeaker belonging to the channel in question. In principle an equal number of microphones and loudspeakers is used, therefore. The microphones and loudspeakers are located at such a distance from a stage that the system only amplifies the reverberant field. The attainable reverberation time is dependent on that of the auditorium itself; it is namely multiplied with a certain factor in dependence on the number of channels.
The loudness of the auditorium is enhanced, because the sound level of the reverberant field is amplified. The hum of voices from the audience, the noise of the ventilation system and the like are amplified along with the other sounds, because all the sound present in the reverberant field is received.
The reverberation time is adjustable by selecting the amplification of the channels differently, by which the coloring and the sound level in the reverberant field are changed at the same time; they are coupled, therefore.
Another known electro-acoustic system which makes use of extension of reverberation time by acoustic feedback is the "Assisted Resonance System", called the AR-system hereinafter, supplied by Airo, Great Britain. The AR-system is inter alia installed in the Royal Festival Hall in London, England, and described in the article "Electro-Acoustic Means of Controlling Auditorium Acoustics" published in Applied Acoustics 0003-682x, 1988 and in the literature mentioned in said article. It is also a multi-channel system whereby, in contrast with the MCR-system, each channel is only active in a frequency bandwidth of 2-5 Hz, by placing each microphone in an acoustic (so-called Helmholtz) resonator. In this way the acoustic feedback in a channel may be high before instability occurs. As a result a single channel realizes a significant extension of the reverberation time in the narrow frequency band in question. In the Royal Festival Hall in London the system consists of 172 channels, always a single channel for a frequency band width of 2-5 Hz, and therewith influences the reverberation time in the frequency range between 58 and 700 Hz.